The present invention relates generally to a method and device for measuring the airflow exiting the duct system of an HVAC system with a fan powered by a controlled external energy source and a flow measurement device.
Airflow through an HVAC system flows into the system through return grilles or outdoor air intakes, is pushed or pulled by the fan, and then is forced through a duct system, exiting through a supply register or grille into a room to provide cooling, heating or ventilation. It is often desirable to measure the airflow out of or into the duct system to determine if the desired flow is being provided to or removed from each room or zone.
Airflows entering or exiting grilles are commonly measured using either a pressure-measurement grid that attempts to determine average velocity or a flow measurement device including an impeller in which the speed of the impeller is an indication of the speed of the airflow.
There are several drawbacks to both of these techniques. Pressure measurement grids suffer from inaccuracies that are caused by spatially non-uniform velocities. Empirical register dependent correction factors have been employed to account for non-uniform velocity biases with pressure measurement grids, however, the use of these correction factors is cumbersome and unreliable in the field applications. Flow straightening elements for pressure-measurement grids have also been employed to eliminate the need for correction factors and improve accuracy, but create excessive flow resistance.
The process of measuring the airflow with the impeller-based flow measurement device also creates flow resistance, impeding the airflow being measured. With conventional flow measurement devices, the impeller is spun by energy in the airflow that is being measured, which means that the impellear changes the flow being measured. Another problem sometimes associated with the prior art impeller-based flow measurement devices is that the measurement of the airflow is biased if the airflow exiting the duct system has a swirled profile. Flows have also been measured in research settings using a flow capture hood and a remote calibrated fan.
Hence, it would be beneficial to employ a flow measurement device that does not impede the airflow being measured and that accommodates for non-uniform or swirled airflow profiles.